Wood and wood products utilized in a variety of construction applications are frequently structurally degraded by the action of termites, ants, other boring insects and wood decaying microorganisms. Typically, these wood degrading and decaying organisms migrate to wood structures via the surrounding soils. This migration may occur whether the structures rest upon concrete foundations, such as in wooden building construction, or if the structures are in direct contact with soil, for example fence posts, utility poles, railroad cross-ties, pier pilings, wooden docks, wooden supports and like structures.
Present methods of preventing or retarding the advance of these wood degrading organisms include both soil treatment with pesticides and repellent chemicals, treating the wood with chemicals, and fumigation wherein the entire structure may be sealed and a pesticide, insecticide or repellent released. Both soil and fumigation type treatments may release the pesticide, insecticide or repellent to the surrounding atmosphere or move to ground water where it may harm human beings or other living organisms. Disadvantages of these methods of treating soil and/or fumigating include potential ecological and human health concerns as well as the limited time until the fumigant or soil concentration is sufficiently reduced in concentration to permit ingress of wood degrading organisms.
Although many pesticides and repellent are known to be effective against the action of wood destroying organisms, their effectiveness often declines over time as they are dissipated into the surrounding environment (soil or atmosphere) or are degraded either chemically or biologically. To retain their effectiveness, these insecticides must therefore be repeatedly applied at regular intervals ranging from every few days to a few months or a few years. Alternatively, if these pesticides and repellents are applied in sufficient quantity to be effective over a period of time, the ecological and human health related concerns associated with these chemicals and the unpleasant odors are exacerbated. Furthermore, with the banning of certain chemicals and the introduction of safer shorter half-life compounds, even large amounts of many of these pesticides and repellents may be required over a relatively short time periods, and they will need to be reapplied more often.
A further disadvantage of conventional application methods is that the concentration of pesticides and repellents resulting from a single application starts out well above the minimum concentration necessary for effectiveness, but decreases rapidly. Within a relatively short period of time that concentration drops below the minimal effective level necessary to maintain a barrier to the invasion of wood destroying organisms.
To overcome these problems, a number of techniques for the controlled release of chemicals such as insecticides have been proposed in recent years. These methods employ polymer matrices and microcapsules used to contain insecticide and allow the slow release of the pesticides and repellents over extended time periods. One such scheme is found in U.S. Pat. No. 4,400,374 to Cardarelli which discloses the use of polymer matrices generally made of polyethylene, polypropylene, ethylene vinyl acetate, polyamide, polystyrene, polyvinyl acetate, or polyurethane to control the release of insecticides such as the insecticide commercially available under the tradename Dursban. The polymer matrices disclosed in U.S. Pat. No. 4,400,374, incorporate porosigen and a porosity reducing agent which upon contact with soil moisture or an aqueous environment dissolves the matrix.
Similarly, Cardarelli U.S. Pat. No. 4,405,360 relates to a polymer release matrix which can be composed of polyamide, polyurethane, polyethylene, polypropylene, polystyrenes and other polymers. The control release mechanism works in combination with a porosigen to release a herbicide in a moist environment.
A disadvantage of the Cardarelli methods is the necessity of sufficient moisture to dissolve the matrix. Periods of dryness, while extending the life of the matrix, would result in a decrease in the insecticide concentration thereby permitting insects to have access to the wooden structure. In addition, the longevity of the matrix is variable and dependent upon moisture content.
In addition, Wysong U.S. Pat. No. 4,435,383 teaches the use of a controlled release mechanism for insecticides including carbamates, organothiophosphates, organophosphates, perchlorinated organics and synthetic pyrethroids. The release mechanism comprises a hydrophobic barrier that is a polymer prepared from a monomer namely styrene and/or methyl styrene in combination with a monomer selected from one or more unsaturated mono- or di-carboxylic acids.
Martinet U.S. Pat. No. 5,860,266 describes the preparation of construction sites with plastic sheets impregnated with an insecticide.
Another reference, Tocker U.S. Pat. No. 4,282,209 discusses a process for the preparation of insecticide-polymer particles. The insecticide, methomyl, is used to control insects which attack a tobacco, cotton or other agricultural crops. Methomyl is dissolved with polymers such as polyamides, urethanes, and epoxies to provide extended residual insecticidal activity.
A second Tocker patent, U.S. Pat. No. 4,235,872, discloses the use of slow-release insecticide microcapsules having a core of methomyl surrounded by a cover of all-aromatic, uncrosslinked polyurea. In the arrangement disclosed in this patent, methomyl is used to protect vegetables, field crops, and fruit crops.
A sixth reference, Young et al. U.S. Pat. No. 4,198,441, discloses the use of insecticides such as chlorpyrofos, tradenamed Dursban, in a controlled release matrix comprising an organopolysiloxane, a hydrolyzable silane and a hydrolyzable organic titanium compound.
Additionally, Young et al. U.S. Pat. No. 4,160,335 discloses a mode of dispersing insect control substances by applying stripes to sheets of cellophane. The insect control substance which can include Dursban is placed in a polymer as well.
Another method is described in an Australian patent AU-B-82443/91. In this patent, there is described two sheets of plastic drawn from supply rolls. The upper face of the lower sheet and the lower face of the upper sheet are drawn past respective coating rollers which apply a coating of pesticide (e.g. permethrin) in a volatile solvent to the faces of the sheets. The coated faces of the sheets are brought together by passing them between compressive rollers. The coated and pressed sheets are laid under building foundations, or placed around trees or plants to prevent termite attack. Disadvantages of this product and method include (1) delamination of a layer permits rapid escape of the coating, and (2) the coating is not integral to the sheets thereby permitting faster diffusion through the sheets and limiting the effective life.
Coated granules have a pesticide absorbed onto a matrix such as clay and then coated with cross-linked resins which helps slow the release rate. Clay loses or releases pesticide over a short period of at most a few weeks.
In U.S. Pat. No. 5,801,194 to Van Voris et al., (the '194 patent) a controlled release device is disclosed which incorporates insecticide into polymer materials to form a device which may then be placed in and around wooden structures to form an effective exclusion zone lasting several years or more. This is accomplished through the use of a low volatility insecticide within a high density polymer, the combination having a low release rate of the insecticide. While the '194 patent does describe a device which provides long lasting protection against insect penetration, the application of this device to either new or existing structures requires the manual placement of the device in and around those structures. The application of the device in this manner may be time consuming, labor intensive, and expensive. In one embodiment of the '194 patent, the controlled release device is placed into a polyurethane foam to allow the spray application of the device to wooden structures. While this approach does lessen the labor, and thus the cost, associated with placing the device in contact with the structures that need to be protected, it does have other drawbacks. For example, at the time the device is applied to the structure, the insecticide is not adequately dispersed throughout the volume of the polyurethane foam. This results in a lack of protection from invasive insects during the time period required for the insecticide to permeate the polymer and infiltrate the foam. To overcome this drawback, the '194 patent also proposes combining the low volatility insecticide within a high density polymer with a more volatile insecticide within a low density polymer which has a higher release rate. The drawbacks of this combined system include the potential harm to human and other life forms which may occur as a result of their coming into contact with the more volatile insecticide.
Thus, there exists a need for improved methods for applying a pesticide or repellent barrier to wooden structures which provides long lasting and immediate protection from termites, ants, wood boring insects and other wood destroying microorganisms as well as provides a protective barrier against the migration of the pesticide, insecticide or repellent into the environment where it can potentially cause adverse ecological and human health impacts. There exists a particular need for methods for applying such barriers which may be accomplished quickly and inexpensively. Finally, there exists a need for methods which accomplish the foregoing objectives in a manner which minimizes the opportunity for the pesticide, insecticide or repellent to come into contact with humans or other living creatures which may be harmed by such contact.